A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning structure, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) or a metrology or inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.
The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.
The term “patterning device” used herein should be broadly interpreted as referring to a device that can be used to impart a projection beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the projection beam may not exactly correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the projection beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.
Patterning devices may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions; in this manner, the reflected beam is patterned. In each example of patterning device, the support structure may be a frame or table, for example, which may be fixed or movable as required and which may ensure that the patterning structure is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning devices”.
The term “projection system” used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate for example for the exposure radiation being used, or for other factors such as the use of an immersion fluid or the use of a vacuum. Any use of the term “lens” herein may be considered as synonymous with the more general term “projection system.”
The illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens.”
The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.
The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the first element of the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.
Before exposing the substrate, it may be desirable or necessary to correctly align it, e.g. to ensure that the functional features are imaged onto the correct position on the substrate. Complementary alignment marks M1, M2 and substrate marks P1, P2 are present on a mask and substrate respectively, and an alignment system is used to detect alignment. Examples of alignment systems are a conventional through the lens alignment system and also the alignment methods and apparatus described in co-pending European application numbers 02251440 and 02250235.
The marks are commonly on the front side of the substrate, but can also be on the backside of the substrate. Marks on the backside of the substrate are used, for example, when exposure is to take place on both sides of the substrate. This occurs particularly in the manufacture of micro electro mechanical systems (MEMS) or micro opto-electro mechanical systems (MOEMS).
When the substrate marks P1 and P2 are on the back surface of the substrate, they may be re-imaged by front-to-back side alignment optics 22 at the side of substrate W to form an image Pi as shown for P2 in FIG. 2 of the accompanying drawings (P1 would be re-imaged by e.g. another branch of the front-to-back side alignment optics). For each apparatus, the distance between the image Pi and the substrate mark P2 is known, and thus the front-to-backside alignment offset for each branch of the front-to-backside alignment optics is known. The front-to-back side alignment optics, together with the alignment system AS are thus used to determine the relative position of marks on the front side of the substrate to marks on the back side of the substrate. This enables functional features exposed on the front side of the substrate to be correctly lined up with functional features exposed on the backside of the substrate.
Although considerable efforts are made to ensure that the positioning of the substrate during an exposure is as accurate as possible, position errors nevertheless remain. These may be of the order of 10-20 nm in a stepper or 5-10 nm in a scanner and are generally tolerable within an overall overlay budget of perhaps 100-500 nm. Furthermore, since in many cases the positioning errors are found to be systematic and characteristic for stage movement speed and/or direction, if all layers of a device are imaged using the same sequence of stage movement, the positioning errors in each layer may be expected to have the same or similar magnitude and direction, such that the overlay error may be reduced. In many cases, the absolute position of the devices on a substrate is less important than the front-to-backside alignment error, that is the error in position of a layer relative to the layers above and below it, in which case the residual positioning errors can be tolerated.
The printing of devices on both the front and back sides of a substrate (i.e. on both principal surfaces of the substrate) imposes considerably stricter requirements on the absolute positioning accuracy, e.g. because the overlay error between front and backside devices may be up to twice the absolute positioning error.